Electronic circuit interruption devices are currently available that employ digital circuit logic for determining overcurrent conditions and for interrupting a protected circuit when such overcurrent conditions persist for predetermined time intervals. U.S. patent application Ser. No. 626,341, filed June 29, 1984 entitled "Circuit Breaker And Protective Relay Unit" describes one such interrupter and is incorporated herein for purposes of reference. Current transformers are arranged for sensing circuit current through each of the 3-phases of an industrial power system. The transformer secondary currents are continuously sampled within a digital processor to determine the maximum RMS current within each of the 3-phases at any given time. As is well-known in the circuit protection industry, the time overcurrent parameters for interruption are based on the heating affects on the various conductors used throughout the power system. The long time overcurrent condition is believed to be related to the heating of the power bus conductors and some time is allowed for this overcurrent condition to exist before the circuit is interrupted. The short time overcurrent condition is believed to be related to the heating of the electrical junctions between the power bus conductors. Since the so called "contact resistance" at these junctions increases rapidly with increases in temperature the overcurrent condition should be interrupted in a shorter period of time. Finally, the high overcurrent situation caused by a short circuit fault should be interrupted as soon as the fault condition is sensed. This overcurrent duration is described as instantaneous with respect to the long and short time overcurrent delays before circuit interruption is initiated. The current transformers used for sensing current through the protected circuit employ magnetic cores that saturate at high fault current conditions. Although the peak values of current can be accurately determined after current transformer core saturation, the measured RMS current necessary for accurate RMS determination decreases as the true RMS current continues to increase. The peak value of sensed current should be analyzed by the digital processor after current transformer core saturation has occurred therefore, to ensure accurate overcurrent determination. Since peak determination of overcurrent conditions is an approximation of the actual RMS current value, based on the mathematical relation between the peak and RMS current values, peak current determination is less accurate for the long and short time overcurrent determinations and could result in so-called "nuisance" tripping. This affect is described within U.S. patent application Ser. No. 743,337, filed June 10, 1985 and entitled "Frequency Multiplying Circuit", which Application is also incorporated herein for reference purposes and should be reviewed for a good description of the nature of capacitive and inductive circuit elements on the distortion of the RMS current wave form and the resulting inaccuracies that could occur when peak current sampling techniques are employed at the lower current long time and short time overcurrent conditions. RMS current sampling techniques provide an extremely accurate determination of the heating affects on the protected circuit prior to saturation of the current transformer core while peak current sampling techniques provide an accurate representation of the heating affects after the current transformer core has become saturated.
The purpose of this invention is to provide means for determining the occurrence of current transformer core saturation and for instructing the digital processor within an electronic circuit interruption device to time out the shortest pre-selected time delay before initiating circuit interruption procedures.